In offset printing, etc., printing must be performed after converting a continuous-tone image to a halftone image. This is because whereas a continuous-tone image (full-tone image) is an image with gray scale information of continuous tones, in ordinary offset printing, etc., one cannot use a plurality of types of inks that differ in concentration in accordance with the gray scale information. A halftone image is basically a binary image in which parts to which ink is to be attached and parts to which ink is not to be attached are distinguished, and it is thus suited for offset printing, etc. However, even in the case where a continuous-tone image, which has been given as an original image, is converted into a halftone image, the gray scale information must be expressed as the area ratios or densities of halftone dots in the halftone image in order to maintain the gray scale information expressed in the original image. Consequently, the halftone image that is obtained can be a pseudo-tone image (pseudo-gray-scale image) that expresses the continuous tones in a simulated manner.
Generally for conversion of a continuous-tone image, which has been provided as an original image, into a half-tone image while maintaining the gray scale information, a halftone mask, called an “AM screen,” is used. This AM screen is a screen for literally performing amplitude modulation of the tone information (gray scale information) possessed by the continuous-tone image and enables conversion in a manner such that the pixel values of the original image are reflected in the areas of halftone dots and halftone images are obtained which comprise halftone dots that are periodic and clustered. In actual form, an AM screen is an aggregation of data comprising a two-dimensional pixel array and the process of conversion from a continuous-tone image to a halftone image is executed by a computing process by a computer that uses these data. A “halftone mask” comprising two-dimensional pixel array data is called a “screen” since it is analogous in function to a contact screen used in traditional photolithography.
Meanwhile, recently, “halftone masks,” by which halftone images with a dispersed type halftone dot configuration can be obtained, have come to be used and a form of image processing called the FM screening process has come to be carried out. Though the “halftone mask” used in such applications has generally been called a “threshold matrix”, “dither matrix”, “dither mask”, etc., in actual form, the mask is, yet again, data comprising a two-dimensional pixel array. Dither masks that are called “art screens,” in which special design patterns are embedded, have also come to be used. When a conversion process using such an art screen is applied, a continuous-tone image, which is provided as an original image, is modulated by the design pattern, which is embedded in the art screen, to provide a special effect such that the halftone image appears to be embossed with the design pattern.
Though such two-dimensional pixel array data used for conversion of a continuous image into a halftone image by a computational process by a computer are called in various ways, for example, “halftone mask”, “AM screen”, “FM screen”, “threshold matrix”, “dither matrix”, “dither mask”, “art screen”, etc., the unified term, “dither mask” shall be used in the present specification. That is, in this specification, “dither mask” shall refer widely to two-dimensional pixel array data used for conversion of a continuous image into a halftone image by a computational process carried out by a computer.
Though various methods have been proposed for generating such a dither mask, each carries problems to be solved. For example, though the type of dither mask that is generally referred to as an “AM screen” enables a halftone image having periodic and clustered halftone dots as mentioned above, there is a problem that unnatural texture, moire pattern or Rosette pattern is observed. In the type of dither mask that is generally called an “FM screen”, though the above problem will not occur since the periodicity of halftone dots can be eliminated, the occurrences of such problems as poor transfer of ink, lowering of the reproducibility of the gray scale characteristics of the original image, etc., have been indicated. Furthermore, with the type of dither mask that is generally called an “art screen,” since there is a need to embed a special design pattern, the procedure for generation is complicated and requires much labor and time.
Thus an object of this invention is to provide a dither mask generation method and generation device by which a dither mask of the desired characteristics can be prepared in accordance with practical application, and a further object of this invention is to provide dither masks generated by such a generation method, and to provide high-quality halftone images and printed objects prepared use of such dither masks.